I know that feeling. The regenerating armour was more than annoying, plus it was immune to damage while regenerating its armour. Horizon was actually one of the only two times I died in all of my play-throughs. That was playing on insanity, too.
Oh man, I HATE praetorians. I'll shoot and shoot and shoot, then they come out from behind cover so I use Charge, and then I can't get back into hiding fast enough and I get killed. >_<
As soon as one of these showed up I focussed all my fast firing weapons on him and then warped the shit out of him once his shields were down...
And still that takes hours xD